Why men usually like to do FLIRT, even when he already in relationships?

I have read from somewhere that when men are in relationships and they still flirt, they are very insecure about themselves. They need to be reassured that they're still desirable, or they want to prove something to women that they've got what it takes to be attractive, or just plain insecure of their spouse that they want to punish them by being so callously flirtatious with other women. In some societies, there is this "machismo" image being identified with men. They believe that it's okay for men to look around or to have a possibility of finding other potential mates. It's even more vile if women would be found adulterous (I guess as with many societies as well). This idea gives men more confidence to do whatever they want, and one situation is flirting with other women even if they're already married.